There has hitherto been known a nitrogen oxide concentration measurement device for measuring the concentration of nitrogen oxides (NOx) in exhaust gases of, for example, internal combustion engines, using an NOx sensor in which, as disclosed for example in European publication EP 0678740A1, and SAE paper No.960334 pages 137 to 142, 1996, etc., a first measurement chamber communicating with a measured gas side via first diffusion rate-regulating layer and a second measurement chamber communicating with this first measurement chamber via a second diffusion rate regulating layer are formed of oxygen ion conductive solid electrolyte layers, a first oxygen pumping cell and an oxygen concentration measurement cell are formed on the first measurement chamber by each sandwiching a solid electrolyte layer by a pair of porous electrodes, and in which a second oxygen pumping cell is formed on the second measurement chamber similarly by sandwiching a solid electrolyte layer by another pair of porous electrodes.
In this type of the nitrogen oxide concentration measurement device, the current is caused to flow across the first oxygen pumping cell so that an output voltage from the oxygen concentration measurement cell will be a pre-set constant value for applying a pre-set constant voltage across the second oxygen pumping cell while maintaining a constant oxygen concentration in the first measurement chamber for pumping out oxygen from the second measurement chamber by applying a constant voltage across the second oxygen pumping cell. The NOx concentration in the gas under measurement is detected based on the current value flowing in the second oxygen pumping cell.
It is noted that gas components other than NOx, that is oxygen, carbon monoxide or carbon dioxide, are present in the exhaust gases from the internal combustion engines, that is gas under measurement. In the above-described nitrogen oxide concentration measurement device, the inside of the first measurement chamber is controlled by the first oxygen pumping cell to an extremely low oxygen concentration, while a constant voltage is applied across the second oxygen pumping cell in a direction of pumping out oxygen from the second measurement chamber, on the side of the second measurement chamber into which flows the gas controlled to the low oxygen concentration, thereby decomposing NOx in the gas under measurement into oxygen and nitrogen by the catalytic function of the porous electrode constituting the second oxygen pumping cell and extracting oxygen from the second measurement chamber. The pump current flowing at this time through the second oxygen pumping cell is detected in an attempt for measuring the NOx concentration in the gas under measurement without being affected by the other gas components in the gas under measurement.
For accurate detection of the NOx concentration by the above detection method using the above nitrogen oxide concentration measurement device, each cell needs to be activated by heating the sensor to a pre-set activation temperature, such as 800.degree. C. or higher. Thus, a separate heater is provided for heating the sensor.